Of all the printing processes only lithography requires a plate treating system in conjunction with the inking unit. It is easy to understand the reason for this requirement if one keeps in mind the peculiar nature of the lithographic plate. It is planographic, meaning that the image areas are in the same plane as the non-image areas. The image areas are hydrophobic or non-receptive to water, but they accept greasy ink. Meanwhile, the non-image areas are hydrophilic or water receptive and they resist greasy ink. However, in order to have the lithographic press perform properly, both the image and non-image areas of the planographic plate must be treated continuously so that they will retain their individual properties.
The primary functions of the inking system of a lithographic process are in general, (1) to work the ink by conditioning it from what is essentially a plastic state to that of a semi-liquid state, (2) to deposit a uniformly even, thin, film of ink on the image areas of the plate, and, (3) to pick up from the lithographic press any particles of foreign matter and hold them until the entire mechanism is cleaned. Meanwhile, the primary function of the plate treating system or plate dampening system of a lithographic press is to apply an aqueous base, plate dampening solution or fountain solution, to the non-printing areas of the printing plate just prior to the time that the plate comes into contact with the inking system. Thus, the inking rollers of the inking system contact both the image and non-image areas of the printing plate as do the dampening rollers of the plate dampening system which applies the fountain solution to the plate. However, because of the water receptive/non-water receptive characteristics of the non-image/image areas of the plate, the ideal situation is one where the ink tends to adhere only to the image areas of the plate. Accordingly, the image/non-image areas are differentiated only by their affinity toward water and not by any inherent difference toward ink. And therefore, trouble-free lithographic printing requires a delicate balance of the oleophilic and hydrophilic properties in the plate-ink-water system that is governed by maintaining a proper surface free energy difference during printing between the image and non-image surfaces.
On the other hand, there is yet another variable that must be taken into account before the optimum performance of the lithographic press can be achieved. Namely, the characteristics of the different types of paper that are employed in lithography and the paper related printability problems that can and do occur. It has been estimated that between 5-20% of the water in the fountain solution can be transferred from the plate to the paper during the lithographic printing process, and although the absolute amount of water transferred is small, the interaction of the fountain solution with the paper can influence printability in many ways. For instance, the water carrying ability of paper influences the correct ink-water ratio especially around highlight areas. Also, the water resistance of paper is important in avoiding piling, milking and related problems. And, finally, the water-paper binder interaction on the paper surface may locally change the ink holdout, to produce wet repellency and deficient color trapping problems.
When coated paper is printed on the lithographic press other problems can occur. Regular coated paper is not wetted easily by pure water. Therefore, the surface tension of the fountain solution must be lower than pure water to permit it to penetrate more easily into the porous coating layer. The tendency of the fountain solution to penetrate the coating layer and extract ingredients therefrom is dependent on the water resistance, composition and porosity of the coated surface. High acidity or high surfactant content of the fountain solution increases its solubilization potential. With respect to the coating ingredients that can be dislocated by the fountain solution, part are water insoluble components that can be suspended in the fountain solution to cause piling on the blanket. They may also be deposited directly on the plate to promote physical wear of the plate. On the other hand, part of the extracted material is water soluble and contains components which can promote ink-in-water emulsification by reducing the interfacial tension at the water/ink interface. In addition, the extracted material can absorb directly onto the printing plate to produce scumming if the water receptivity of the non-image areas is reduced, or plate blinding if the ink receptivity of the image areas becomes impaired. Thus it may be seen that the paper itself plays an important and hitherto unstudied role in the successful action of the lithographic process.
Heretofore, fountain solutions have comprised either tap water, distilled water, dilute aqueous solutions of gum arabic, or relatively complex formulations of water and alcohol and/or other organic solvents. Also it has been proposed heretofore to add glycerine and/or other deliquescents and/or humectants to the fountain solution. In some instances, other additives such as collodial silicic acid and phosphonic acid have been used. However, each of the many fountain solutions of the prior art have been formulated for use with a particular type of printing plate and have not been suitable for use as a universal solution.
As examples of some of the prior art fountain solutions, reference may be made to the following U.S. patents which teach the use of fountain solution formulations containing alcohol and substitutes for alcohol:
______________________________________ 2,250,516 W.H. Wood 3,053,178 P.W. Greubel 3,354,824 R.C. Griffith et al 3,398,002 L.E. Bondurant 3,625,715 S. Nasca 3,679,479 D.A. Ray et al 3,877,372 K.W. Leeds ______________________________________
However, by the present invention, a fountain solution is disclosed which finds application with all types of lithographic printing plates and in all of the known plate dampening systems. Moreover the fountain solution of the present invention finds particularly good application in the Dahlgren dampening system. The Dahlgren dampening system differs from a conventional dampening system primarily by using the inking system itself to carry the fountain solution as well as the ink to the printing plate. In a conventional lithographic dampening system, the fountain solution is applied to the printing plate from a dampening unit that is separate from the inking system. Thus with the Dahlgren system, the critical relationships between the plate-water-ink-paper interactions are more pronounced than with a conventional dampening system. In this respect, the fountain solution of the present invention overcomes inherent problems in the Dahlgren system, provides a pollution free formulation and takes into account paper related printability problems that have not heretofore been investigated.
The fountain solution of the present invention is useful for the printing of C2S, C1S, surface sized, uncoated and newsprint papers on either web fed or sheet fed presses and tends to alleviate such paper related printing problems as wet piling, linting, picking, plate blinding, loss of ink receptivity and ink offsetting with both conventional and Dahlgren dampening systems. These results are achieved with the fountain solution of the present invention by permitting a lower fountain solution feeding rate to the printing nip; by providing a quicker and more uniform ink distribution on the inking rollers; by producing a lower retention of the fountain solution in the ink supply; by providing faster drying of inks which dry by an oxidation mechanism; by producing better blanket release from the paper; and by providing the printing plates with a longer life.
The fountain solution of the present invention is also low in cost and contains components which are generally readily available and in good supply. In this regard, the fountain solutions normally used in the Dahlgren dampening system contain from 15-25% by volume of an alcohol such as isopropanol. However, isopropanol is from time-to-time in short supply, its cost has been rising steadily and it presents a potential threat to the press room environment. To offset the problems attendant to the use of isopropanol in the Dahlgren dampening system, several so-called alcohol substitutes have been proposed. Unfortunately, most of these alcohol substitutes present other problems including excessive foaming, uneven ink distribution on the inking rollers and ink stripping. Of those that tend to perform satisfactorily on the press, a higher fountain solution feeding rate is required and special adjustments of the distribution rollers must be made. On the other hand, the fountain solution of the present invention does not use alcohol in its formulation and it takes into account the various paper surface properties and problems set forth hereinbefore in relation to the fountain solution-paper-ink interaction in the printing nip.